1. Field of the Invention
The present invention relates to a method of manufacturing a substrate having a cavity (recess) for installing an electronic component therein.
2. Description of Related Art
With mobile devices such as mobile phones and smartphones having a smaller profile, there is strong demand for reducing the thickness of the substrate having an electronic component installed therein, and one method of realizing reduced thickness is to provide a cavity (recess) (refer to Patent Document 1 below, for example). That is, by installing an electronic component inside the cavity, it is possible to reduce the thickness of the substrate having installed therein the electronic component.
Examples of methods to provide the substrate with a cavity include: (M1) forming a cavity by mechanical cutting; (M2) forming a cavity by chemical etching; and (M3) forming a cavity by stacking a plurality of layers having penetrating holes already formed therein on a base layer an adhesive therebetween, the plurality of penetrating holes integrally forming a continuous cavity.
The previously mentioned method M1 can be performed whether the material in which the cavity is to be formed is metal or synthetic resin. However, if the cavity is formed such that the thickness below the bottom surface is very thin (100 μm or less, for example) then there is a risk that stress resulting from cutting will result in damage such as cracking in the bottom surface. Also, cutting involves a dimensional tolerance of at least ±tens of μm, and thus, there is a tendency for variation to occur in the cross-sectional dimensions and depth of the cavity, and it is difficult to create a cavity with smooth inner walls and bottom surface.
The method M2 allows for higher precision (dimensional tolerance of ±less than 10 μm) than the method M1. However, if the material to be etched is synthetic resin, the time taken to etch is generally longer than if the material were metal. Also, the etchant used for the synthetic resin generally depends on the type of synthetic resin, but this method is unrealistic due to some types of resins (such as epoxy resin) requiring dangerous substances such as concentrated sulfuric acid or chromic acid.
The method M3 does not require the types of processes performed in the method M1 or the method M2. However, it is difficult for the cross-sectional dimensions of the penetrating holes formed in the respective layers to match perfectly, and it is difficult to stack the respective layers such that the penetrating holes align without being offset, and thus variation tends to occur in the cross-sectional dimensions of the cavity, which makes it difficult to have a cavity with smooth inner walls. Also, if an adhesive is used when stacking the respective layers, there is a risk that some of the adhesive seeps out to the bottom surface of the cavity when the layers are stacked, which results in variation in depth of the cavity or loss of smoothness in the bottom surface.